From FIGS. 1 and 4 of German Patent Application DE 44 45 401 A1, a vehicle hydraulic brake system is known that has a master cylinder which has a housing and in it a bore in which a first piston can be displaced as a function of the depression of a brake pedal. A second piston is disposed displaceably on a side of the first piston remote from the brake pedal. The first and second pistons between them define a first pressure chamber, which communicates with a first connection embodied on the housing and with a second connection. The second piston on a side remote from the first pressure chamber defines a second pressure chamber, which communicates with a third connection embodied on the housing and with a fourth connection. The first piston blocks off the communication between the first connection and the first pressure chamber, and the second piston blocks off the communication between the third connection and the third pressure chamber--in each case when the brake pedal is depressed. A supply container is provided for storing pressure fluid and for communicating with the third connection of the master cylinder. A valve assembly is provided for opening or closing the communication between the first connection of the master cylinder and the supply container. A pump device, which has a suction connection communicating with the supply container and a pressure connection communicating with the second connection of the master cylinder and which aspirates the pressure fluid from the supply container and pumps it to the second connection. A plurality of wheel brake cylinders are provided for associated wheels, in order to brake these wheels, the brake cylinders are divided into a first group of wheel brake cylinders and a second group of wheel brake cylinders; having first brake pressure modulation valve assemblies, which are disposed between the second connection of the master cylinder and the first group of wheel brake cylinders; having second brake pressure modulation valve assemblies, which are disposed between the fourth connection of the master cylinder and the second group of wheel brake cylinders. A control unit, in order to actuate the valve assembly for opening or closing the communication between the first connection of the master cylinder and the supply container an agreement with an operating mode selected from among an opening operating mode and a closing operating mode is provided. The pump device is embodied as a high-pressure pump, whose feed pressure is limited by a pressure limiting valve to a pressure that is at least as high as a maximum brake pressure to be supplied to at least one wheel brake cylinder. As a result, this hydraulic vehicle brake system is capable without actuation of the brake pedal of generating brake pressures for traction control, also known as drive slip control, by conditional braking of at least one drivable wheel. The hydraulic vehicle brake system of FIGS. 1 and 4 have the disadvantage that aside from normal braking by brake pedal actuation, it is arranged for only the traction control mode, and that the high pressure that can be furnished by this pump device requires a first connection on the master cylinder that can withstand this high pressure. Accordingly, it is not possible to use a normal master cylinder with the known connections, in which as connecting elements to the respective supply container merely elastic rubber rings are placed. Instead of the communication via this kind of elastic rubber ring, a screw connection would have to be provided, for instance, which is for instance identical to screw connections for the second connection and the fourth connection. It is advantageous, however, that by means of the second connection the pump device builds up a pressure in the pressure chamber that by means of the second piston also produces a pressure rise in the second pressure chamber and this pressure rise is available through the fourth connection to the second brake pressure modulation valve assemblies. Accordingly--beginning at one outlet of the pump device--only one hydraulic communication with the master cylinder is required.
A further hydraulic vehicle brake system known from FIG. 5 of DE 44 45 401 A1 omits the pump device, present in the above-described vehicle brake system, by using so-called return feed pumps of an anti-lock device; the return pump associated with the second connection of the master cylinder supplies itself with pressure fluid from the supply container through a suction line, into which a normally blocking, electrically controllable 2/2-way valve is incorporated. However, this has the disadvantage that incorrect control or a defect of this 2/2-way valve causes an opening of the brake circuit originating at the second connection of the master cylinder, with the result that during the anti-lock mode, pressure fluid can escape to the supply container, and the brake pedal can sink deeper. Clearly this can have the result that an intrinsically available reserve stroke of the first piston will be used up, with the loss of braking power. Another disadvantage is that this return pump supplies itself with pressure fluid through the aforementioned suction line and the aforementioned 2/2-way valve only incompletely, for instance if at winter temperatures this temperature fluid is viscous, or if at summer temperatures the development of vapor bubbles from an air draft cannot be precluded. Inadequate filling of this self-supplying return pump naturally causes a defective pumping capacity and consequently a disadvantageously slow rise in brake pressure or an inadequate rise in brake pressure. A slow rise in brake pressure in the traction control mode is disadvantageous especially when traveling over ice or snow.
A vehicle brake system known from German Patent Application DE 42 26 646 A1 is arranged to avert the danger of wheel locking in braking and also for automatic braking, for instance for traction control, for distributing braking force between the front and rear wheel brakes, and/or for individual braking of at least one of the vehicle wheels for the sake of stabilizing the vehicle if there is a tendency to oversteer, for the sake of obtaining vehicle steerability if there is a tendency to understeer, for the sake of the best possible adherence in terms of driving dynamics to the vehicle motion curve specified by the steering angle, as a result of the fact that if there is a tendency to oversteer and understeer or to depart from the predetermined path curve, this curve is corrected by braking or by boosted braking of at least one wheel. This vehicle brake system likewise has a master cylinder with four connections, a supply container, two brake circuits connected to the master cylinder for a plurality of wheel brake cylinders of a plurality of wheels, and brake pressure modulation valve assemblies disposed between them with multiposition valves and with one pump per brake circuit, which pump functions in operation to reduce the danger of wheel locking or in the automatic braking mode furnishes pressure at least at a level sufficient for braking; for automatic braking, valve assemblies are provided between the pressure outlets of these pumps and the respective connections of the master cylinder which are controlled for automatic braking from their basic position into control positions so that quantities of pressure fluid will flow out of these pumps to the master cylinder only once projected pressures are reached. The aforementioned valve assemblies accordingly have the function on the one hand of a multi-position valve which is open for normal braking and on the other, the function of a safety valve that protects the applicable pump against an overload. Inlets of the two pumps are supplied with pressure fluid from the supply container of the master cylinder by a common charge pump unit, through electrically controllable multiposition valves. To that end, the charge pump unit furnishes a pressure on the order of magnitude of 5 bar, for instance. The charge pump unit includes a charge pump, a pressure limiting valve connected to the outlet of the pump, and a throttle also connected to this outlet, which throttle communicates with the supply container and accordingly with one inlet of the charge pump. It is advantageous that the charge pump needs to generate a pressure only on the aforementioned order of magnitude of 5 bar and therefore produces little noise, absorbs little drive output, and is accordingly inexpensive. It can be considered disadvantageous that if the supply container is inadequately filled and under the influence of acceleration or if the pressure fluid is foaming, the charge pump will aspirate air, and some of this aspirated air may force its way into both pumps and thus into both brake circuits, causing volumetric elasticities there. Such volumetric elasticities are known to be capable of causing inadequate braking action.